The present invention relates to a high speed impact printer.
Printers comprising resilient type wheels are becoming increasingly popular in the art due to their many advantages. These type wheels comprise a rotary hub with a plurality of resilient arms extending therefrom in a spoke-like manner. Type elements are provided at the end portions of the arms. These type wheels can be fabricated at very low cost and provide high speed, effective printing.
The type wheel is rotated until the selected type element is in a printing position relative to a platen around which paper is wound. A hammer is driven to engage the selected type element and drive the same to impact against the paper through an inked ribbon, thereby printing the desired character. The selected arm resiliently bends during this process. After impact the type element and hammer return to their original positions due to the resilience of the arm and a return spring for the hammer.
At impact, the hammer intersects a plane containing the non-selected type elements. If the type wheel were rotated at this time the arm next to the selected arm would strike the hammer causing a jam, breakage of one or more arms or both. The hammer must be retracted to such an extent as to clear the type wheel before the type wheel can be rotated for selection of the next character for printing.
Although such printers are often operated to print only one sheet of paper, it is sometimes desired to make several copies by means of interspersed sheets of ordinary paper and carbon paper. The distance between the type wheel and the platen is adjusted by means of a lever according to the number of sheets or copies to be printed. Since a large number of sheets has a cushioning effect on the type wheel and hammer thereby absorbing impact energy, the hammer returns more slowly after impact when a large number of sheets are printed. Thus, rotation of the type wheel must be inhibited for a longer length of time after impact when a large number of sheets are printed.
Prior art printers are set up to inhibit rotation of the type wheel for the maximum length of time it could possibly take the hammer to clear the type wheel after impact, which necessarily corresponds to the maximum number of sheets which can be printed by the printer. This constitutes a waste of operating time where only one sheet or an intermediate number of sheets is printed since the printer remains idle for a length of time equal to the difference between said maximum length of time and the actual length of time it takes the hammer to clear the type wheel. In other words, the printer operates at its maximum possible speed only when the maximum number of sheets are being printed, and is unnecessarily prevented from being speeded up where less than the maximum number of sheets are being printed.